The present invention relates to vehicle traction drives including right and left two-speed, hydraulic motors and more specifically relates to speed controls for such motors.
Vehicles such as crawler-type excavators are commonly provided with traction drives including separate, two-speed hydraulic motors for driving the tracks at the opposite sides of the vehicle. In a known system for controlling the shifting of the motors between their two speeds there is provided a two-position solenoid speed control valve selectively actuated by an "on-off" speed control toggle switch, the motors being conditioned to operate in slow speed when the switch is "off" and the valve is in a deenergized position and being conditioned to operate in high speed when the switch is "on" and the valve is in an energized position. When driving these vehicles across a field at high speed it sometimes becomes necessary to realign the vehicle to keep it on a desired course and, since turning requires higher torque than is required for proceeding straight, it is often necessary to downshift the motors in order to make the realignment. Since it may be necessary to make several such adjustments if the vehicle is being driven very far, operators are often annoyed at having to constantly manipulate the speed control switch.